[TypeScript] Implement oneOf type resolution without discriminator

[ksvirkou-hubspot]

Summary

• Implements oneOf type resolution for TypeScript generator when discriminator is not present
• Adds OneOfClass with findMatchingType() method to determine correct type at runtime
• Enhances ObjectSerializer to use type deduction when discriminator is unavailable
• Includes comprehensive tests validating both discriminator and non-discriminator scenarios

Why

Previously, the TypeScript generator could only resolve oneOf types when a discriminator property was defined in the OpenAPI spec. This limitation meant that valid oneOf schemas without discriminators would fail at runtime.

This change enables automatic type resolution by checking which schema matches the provided data based on required fields, making the generated TypeScript clients more robust and easier to use for specs that don't define discriminators.

Implementation Details

• Created OneOfClass base class with instanceOf() method to validate objects against schema requirements
• Generated oneOf models now extend OneOfClass and include findMatchingType() to iterate through possible types
• ObjectSerializer checks for findMatchingType() method when discriminator is undefined and uses it for type resolution
• Falls back to original behavior if neither discriminator nor findMatchingType() is available
• Throws descriptive error when type resolution is ambiguous (object matches multiple or no schemas)

Tests

Added integration tests at samples/openapi3/client/petstore/typescript/tests/one-of/:

• Test spec: modules/openapi-generator/src/test/resources/3_0/typescript/oneOf.yaml defines two endpoints with oneOf responses
• Without discriminator test: Validates that PetResponse (oneOf without discriminator) correctly deserializes to Cat based on required fields (name, petType)
• With discriminator test: Validates that PetDiscriminatorResponse (oneOf with discriminator) correctly deserializes to Dog using discriminator property
• Tests use Mocha + Chai with Nock for HTTP mocking
• Run with: npm test in the test directory

Related issue: #19868

PR checklist

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[ksvirkou-hubspot]

The PR contains breaking change to generated TypeScript code, attributeTypeMap structure changed: Added required: boolean field to all model attribute type maps
- Before: Array<{name: string, baseName: string, type: string, format: string}>
- After: Array<{name: string, baseName: string, type: string, format: string, required: boolean}>
I can create method in model getRequiredFields() or create PR agains 8.0.0
Please check the PR and provide code review

[macjohnny]

thanks for the changes!

[ksvirkou-hubspot]

Could you please check the PR again?
I've added significant changes.
Do I need to create a new PR against the 8.0 branch?
@macjohnny